Method and apparatus for cleaning waste flue gases

ABSTRACT

A waste flue gas cleaning method and an apparatus for practicing said method, in which a reactant solution is sprayed into a waste flue gas introduced into a spray tower, whereby a toxic component in the flue gas is caused to be absorbed in and react with the droplets of the sprayed reactant solution, and the water contained in the droplets is vaporized and the reaction products are dried into solid particles by the sensible heat of said flue gas. A series of the above operation is accomplished within the spray tower while maintaining the outlet gas temperature always above the dew point of the flue gas, and the solid particles of the reaction products are removed from the flue gas by an after dust collector.

This is a continuation of application Ser. No. 197,376, filed Nov. 10,1971, and now abandoned.

This invention relates to a method of and an apparatus for cleaningwaste flue gases by removing therefrom toxic gases such as SO₂, NO₂, NO,H₂ S, HF, F. etc.

The object of the present invention is to provide a waste flue gascleaning method which can be practiced by a highly economical, verysimple apparatus, i.e. a method which comprises the steps of spraying areactant solution in the form of a fine mist into a hot waste flue gas,causing the toxic components contained in said waste flue gas to beabsorbed by and react with said reactant solution, drying the waterpresent in the droplets of said reactant solution by the sensible heatof the waste flue gas and collecting the reaction products in the formof solid particles, a series of said steps of operation continuouslybeing carried out within a single spray drying tower in a short periodof time in a manner to prevent a temperature lowering and humidityincrease of the waste flue gas caused by the vaporization of the watercontained in the sprayed reactant solution and constantly maintainingthe humidity of the flue gas released into the atmosphere above the dewpoint thereof, thereby to prevent the atmospheric pollution otherwisecaused by the flue gas.

According to the present invention it is also possible to preventsecondary public nuisance caused by a large amount of waste waterdischarged from apparatus by which conventional wet-type methods arepracticed. Further, according to the invention it is possible, inrecovering valuable substances from the waste water, to omit acondensing or extracting operation, which is highly advantageous fromthe standpoint of economy.

The present invention will be described in detail hereunder withreference to the accompanying drawings. In the drawings,

FIG. 1 is a schematic view illustrating the waste flue gas cleaningmethod according to the present invention; and

FIG. 2 is a system diagram showing an embodiment of the apparatusaccording to the invention.

Referring first to FIG. 1 there are shown the principle of the wasteflue gas cleaning method of the invention and the brief construction ofan apparatus for practicing said method.

A contaminated waste flue gas 1 is induced into an absorbing andvaporizing tower 3 through an inlet duct 2. On the other hand, areactant solution consisting, for example, of an aqueous caustic sodasolution of a consistency of about 10 % supplied through a duct 5 alongwith compressed air 14 is sprayed from nozzles 6 into the waste flue gasin the tower 3.

When the waste flue gas is introduced the sprayed mist zone via guidevanes 4, toxic components contained therein contact the droplets of thesprayed solution and are absorbed thereby. In the droplets, thefollowing reactions take place:

Sulfur dioxide:

    2NaOH+SO.sub.2 →Na.sub.2 SO.sub.3 +H.sub.2 O        (1)

    Na.sub.2 SO.sub.3 +1/2O.sub.2 →Na.sub.2 SO.sub.4    (2)

    Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O→2NaHSO.sub.3 (3)

    2NaHSO.sub.3 +O.sub.2 →2NaHSO.sub.4                 (4)

    Na.sub.2 CO.sub.3 +SO.sub.2 →Na.sub.2 SO.sub.3 +CO.sub.2(5)

Nitrogen oxides:

    Na.sub.2 SO.sub.3 +1/2NO.sub.2 →Na.sub.2 SO.sub.4 +1/4N.sub.2(6)

    2NaOH+NO.sub.2 →Na.sub.2 NO.sub.3 +H.sub.2 O        (7)

    2NaOH+NO+NO.sub.2 →2NaNO.sub.2 +H.sub.2 O           (8)

Hydrogen sulfide:

    NaOH+H.sub.2 S→NaHS+H.sub.2 O                       (9)

    NaHS+NaOH→Na.sub.2 S+H.sub.2 O                      (10)

Fluorine and hydrogen fluoride:

    NaOH+HF→NaF+H.sub.2 O                               (11)

    2NaOH+F.sub.2 →2NaF+1/2O.sub.2 +H.sub.2 O           (12)

The droplets of the reactant solution having the toxic componentsabsorbed and dissolved therein as a result of the above reactions fallin a zone 7 within the tower 3 while being suspended in the waste fluegas, and in this case the water of the droplets is quickly vaporized bythe sensible heat of the waste flue gas. The time required for thevaporization of water is within a few seconds.

In the vicinity of a zone 8 disposed downstream of the tower 3, thereaction products are formed into solid particles. These solid particlesare separated and collected by an after dust separator 9, such ascyclone, electrostatic precipitator or bag filter while the cleaned fluegas is discharged through an outlet duct 15.

The solid particles of the reaction products collected in the after dustseparator 9 are discharged to the outside from a discharge duct 13 viadischarge means 11 such as a rotary valve or dual air-lock damper, andreused as such as a valuable substance, or regenerated into caustic sodaand other valuable substances. The caustic soda thus produced can berecycled through the apparatus for use.

A part of the solid particles of the reaction products formed in thetower 3 through the steps of absorption, reaction and vaporization isdischarged from the bottom of the tower to the outside through adischarge duct 12 via discharge means 10.

As may be understood from the foregoing description, the presentinvention consists in a waste flue gas cleaning method which comprisesspraying a reactant solution in the form of a fine mist into a wasteflue gas introduced into a tower, causing toxic components in the wasteflue gas to be absorbed by and react with the mist of the reactantsolution, completely drying the reaction products into solid particleswithin the tower by the sensible heat of the waste flue gas, andseparating and collecting the solid particles of the reaction products.

In practicing the method of this invention, the temperature of the wasteflue gas lowers and the humidity thereof increases incident to thevaporization of the water contained in the sprayed solution, with theresult that the cleaning efficiency is degraded. In order to achievesatisfactory cleaning, it will be effective to control the amount ofwater contained in the sprayed reactant solution and the amount of saidreactant solution so as to maintain the temperature of the waste fluegas discharged from the apparatus above the dew point thereof.

A practical arrangement to achieve this will be described with referenceto an embodiment of the apparatus of the invention shown in FIG. 2. Theconcentration of a toxic gas (e.g. SO₂) is detected by a gasconcentration decoder 16 and converted into an electric signal, and theflow rate of the waste flue gas is detected by a gas flow rate detector17 and converted into an electric signal. The electric signals generatedby the gas concentration detector 16 and the gas flow rate detector 17are supplied to a calculator, which in turn generates an electric signalproportional to the product of said two electric signals. The flow rateof the reactant solution (e.g. a caustic soda solution) of apredetermined concentration is measured by a flow meter 20 and convertedinto an electrical signal. The electric signals from the flow meter 20and the calculator 18 are supplied to a constant rate presetter 19, inwhich the ratio of said electric signals is compared with a presetvalue, and said constant rate presetter 19 generates an electric signalrepresentative of the balance of the value of said ratio and said presetvalue. A control valve 21 is actuated in response to the electric signalfrom the constant rate presetter 19 to regulate the flow rate of thereactant solution, so that the said reactant solution will be suppliedin an amount equivalent to the amount of the toxic gas. Thus, thereactant solution can be supplied at a predetermined ratio according tothe varying amount of the flue gas and concentration of the toxic gas atthe inlet duct 2 of the tower 3, but the temperature of the outlet gasfluctuates. In order to maintain the outlet gas temperature constantlyabove the dew point thereof, the outlet gas temperature or humidity isdetected by a detector 22 and converted into an electric signal. Theelectric signal from the outlet gas temperature or humidity detector 22is supplied to a presetter 23 in which it is compared with a presettemperature or humidity value, and said presetter 23 generates anelectric signal representative of the balance of said two values. Acontrol valve 24 is actuated by the electric signal from the presetter23, to regulate the flow rate of water to be mixed with the reactantsolution. The flow rate of the reactant solution whose consistency hasthus been adjusted is detected by a flow meter 25 and converted into anelectric signal, while the flow rate of compressed air for spraying thereactant solution is detected by a flow meter 26 and converted into anelectric signal. The electric signals from the flow meters 25, 26 aresupplied to presetter 27, in which the ratio of said electric signals iscompared with a preset value, and an electric signal representative ofthe balance of said two values is generated by said presetter 27. Acontrol valve 28 is actuated in response to the electric signal from thepresetter 27 to regulate the flow rate of air, so that a preset ratio ofair to the reactant solution may be supplied.

The material solution of the reactant solution to be sprayed is suppliedfrom a tank 33 by means of a pump 34. In order to maintain theconcentration of the material solution at a predetermined level, thereare provided a flow meter 29 for measuring the flow rate of water and aconstant rate feeder 32 for metering and feeding a reactant (e.g.caustic soda) into the tank 33 at a rate proportional to the flow rateof water. When the reactant solution has reached a predetermined levelin the tank 33, the level of the reactant solution is detected by aliquid level detector 30 disposed in said tank, and a valve 31 is closedin response to a signal from said level detector 30.

In the manner described above, the absorption, reaction, vaporizationand drying of the toxic components are continuously carried out in asingle and the same spray tower, and the reaction products are collectedin the form of solid particles, while maintaining the outlet gastemperature always above the dew point thereof, even when the inlet gastemperature, the amount of the waste flue gas and the concentrations ofthe toxic components fluctuate.

Now, an example of the present invention will be illustrated hereunder,in which SO₂ contained in a waste flue gas was removed by the method ofthe invention.

Temperature of the waste flue gas: about 150° C.

Water content in the waste flue gas: about 10 % by volume

Concentration of SO₂ : about 1500 p.p.m.

Concentration of aqueous caustic soda solution sprayed: about 11.5 %

Amount of the solution sprayed: 0.03 l/m³ of gas

The method of the present invention was practiced under the conditionsset forth above. The desulfurization degree was about 80 %; the gastemperature at the outlet opening of the cyclone separator was about105° C. (the dew point was about 55° C.); and the pressure loss wasabout 50 mmH₂ O. The reaction products were collected as solid particlesof Na₂ SO₃ and Na₂ SO₄ from the bottom of the absorbing and vaporizingtower and the cyclone separator.

The cyclone separator used in the embodiment of the present inventionmay be combined with or substituted by a bag filter or electrostaticprecipitator.

Besides caustic soda, other alkaline reactant solutions and slurry ofcalcium hydroxide may be selectively used depending upon the type andcharacteristic of the toxic gas desired to be removed. In operating theapparatus of the invention, it is also effective to heat the wall of theabsorbing and vaporizing tower.

In summary, the apparatus of the present invention comprises anabsorbing and vaporizing tower having an inlet and an outlet for a wasteflue gas and equipped with spray means for spraying a reactant solutionin an atomized state into the waste flue gas introduced into said tower,detectors for detecting the flow rate of the inlet gas and theconcentration of a toxic component contained in the waste flue gas atthe inlet of the tower respectively, and means for supplying thereactant solution to said spray means after adjusting the ratio betweenwater and the reactant, said reactant solution supplying means beingoperative in response to signals from said detectors and a gastemperature or humidity detector provided at the outlet of said tower sothat the reaction products produced within said tower may be formed intosolid particles at a temperature above the dew point of the waste fluegas. The waste flue gas cleaning apparatus of the invention isadvantageous over conventional dry methods such as an activate charcoaladsorption method, in that it can be operated with a pressure loss notgreater than 100 mmH₂ O and is substantially small in volume and simplein construction. Therefore, the apparatus is excellent also from thestandpoint of economy.

Another advantage is that since the reaction products can be collectedin the form of completely solidified particles, the cost of processingthe waste water can be reduced. These solid particles can be effectivelyused as by-products.

What is claimed is:
 1. A method of cleaning waste flue gas, whichcomprises the steps of:(a) spraying an aqueous reactant solution into awaste flue gas introduced into a tower, said waste flue gas containing awater content of about 10% by volume, (b) absorbing and reacting toxicgas in the flue gas in and with droplets of the sprayed reactantsolution, (c) vaporizing water from the droplets and drying the reactionproducts by the sensible heat of the waste flue gas, all of the abovesteps of operation being completed within the tower. (d) separating andcollecting the reaction products in the form of solid particles, (e)controlling the temperature of the waste flue gas discharged from thetower into the atmosphere above the dew point, such that atmosphericpollution by the discharged flue gas is prevented, by controlling theamount of water in said aqueous reactant solution and the amount of saidreactant solution; said step (e) being carried out by the steps of: (f)detecting the concentration of toxic gas in said waste flue gas andgenerating a first electrical signal, (g) detecting the flow rate ofsaid waste flue gas and generating a second electrical signal, (h)regulating the flow rate of said reactant solution as a function of saidfirst and second electrical signals to supply said reactant solution inan amount at least equivalent to the amount of said toxic gas, (i)detecting the temperature of the discharged waste flue gas andgenerating a third electrical signal, (j) regulating the flow rate ofwater mixed with said reactant solution as a function of said thirdelectrical signal, and (k) supplying said reactant solution to which theflow rate of water has been regulated to said waste flue gas in saidtower.
 2. The method of claim 1, wherein said reactant solution is anaqueous caustic soda solution.
 3. The method of claim 1, wherein saidreactant solution is an aqueous alkaline solution.
 4. The method ofclaim 1, wherein said reactant solution is sprayed into said waste fluegas in the form of a fine mist.
 5. The method of claim 1, wherein saidreactant solution is sprayed with compressed air.
 6. The method of claim1, wherein said reaction products are completely dried into solidparticles.
 7. The method of claim 1, wherein the step of separating andcollecting includes at least one of cyclone separating, bag filtering,and electrostatically precipitating.
 8. The method of claim 1, whereinsaid step of separating and collecting is by electrostaticallyprecipitating the reaction products.
 9. The method of claim 1, whereinthe step of controlling the temperature includes automaticallycontrolling the amount of water in said reaction solution and the amountof reaction solution sprayed into the waste flue gas.
 10. The method ofclaim 1, wherein the temperature of the waste flue gas discharged fromthe tower is controlled to about 105° C.
 11. The method of claim 1,wherein a series of said steps (a) through (d) are continuously carriedout within a short period of time.
 12. The method of claim 11, whereinsaid series of continuously carried out steps (a) through (d) areperformed within a single spray drying tower.
 13. The method of claim 1,wherein desulfurization of said waste flue gas is carried out up to atleast 80%.
 14. The method of claim 1, wherein said toxic gas is sulfurdioxide.
 15. A method of cleaning waste flue gas, which comprises thesteps of:(a) spraying an aqueous reactant solution into a waste flue gasintroduced into a tower, said waste flue gas containing a water contentof about 10% by volume, (b) absorbing and reacting toxic gas in the fluegas in and with droplets of the sprayed reactant solution, (c)vaporizing water from the droplets and drying the reaction products bythe sensible heat of the waste flue gas, all of the above steps ofoperation being completed within the tower, (d) separating andcollecting the reaction products in the form of solid particles, (e)controlling the temperature of the waste flue gas discharged from thetower into the atmospheric above the dew point, such that atmosphericpollution by the discharged flue gas is prevented, by controlling theamount of water in said aqueous reactant solution and the amount of saidreactant solution; said step (e) being carried out by the steps of: (f)detecting the concentration of toxic gas in said waste flue gas andgenerating a first electric signal, (g) detecting the flow rate of saidwaste flue gas and generating a second electric signal, (h) comparingsaid first and second electrical signals to generate a third electricalsignal proportional to the product of said first and second electricalsignals, (i) detecting the flow rate of a predetermined concentration ofsaid reactant solution and generating a fourth electrical signal, (j)comparing said third and fourth electrical signals with a predeterminedreference signal and generating a fifth electrical signal, (k)regulating the flow rate of said reactant solution in response to saidfifth electrical signal to supply said reactant solution in an amount atleast equivalent to the amount of said toxic gas, (l) detecting thetemperature of the discharged waste flue gas and generating a sixthelectrical signal, (m) comparing said sixth electrical signal with apredetermined second reference signal to generate a seventh electricalsignal, (n) regulating the flow rate of water mixed with said reactantsolution in response to said seventh electrical signal, (o) detectingthe flow rate of the reactant solution to which the flow rate of waterhas been regulated to generate an eighth electrical signal, (p)generating a ninth electrical signal representative of the flow rate ofair for spraying said reactant solution into said tower, (q) comparingsaid eighth and ninth electrical signals with a predetermined thirdreference signal to generate a tenth electrical signal, and (r)regulating the flow rate of said air in response to said tenthelectrical signal, thereby supplying said reactant solution to saidwaste flue gas in said tower.
 16. The method of claim 5, wherein theconcentration of material for said reactant solution is maintained at apredetermined amount.
 17. The method of claim 15, wherein said toxic gasis sulfur dioxide.